Networking Properties

There are a few standard system properties used to
alter the mechanisms and behavior of the various classes of the
java.net package. Some are checked only once at startup of the VM,
and therefore are best set using the -D option of the java command,
while others have a more dynamic nature and can also be changed using
the System.setProperty() API. The purpose of this document is to list
and detail all of these properties.

If there is no special note, a property value is checked every time it is used.

IPv4 / IPv6

java.net.preferIPv4Stack (default: false)
If IPv6 is available on the operating system the
underlying native socket will be, by default, an IPv6 socket which
lets applications connect to, and accept connections from, both
IPv4 and IPv6 hosts. However, in the case an application would
rather use IPv4 only sockets, then this property can be set to true.
The implication is that it will not be possible for the application
to communicate with IPv6 only hosts.

java.net.preferIPv6Addresses (default: false)
When dealing with a host which has both IPv4
and IPv6 addresses, and if IPv6 is available on the operating
system, the default behavior is to prefer using IPv4 addresses over
IPv6 ones. This is to ensure backward compatibility, for example
applications that depend on the representation of an IPv4 address
(e.g. 192.168.1.1). This property can be set to true to
change that preference and use IPv6 addresses over IPv4 ones where
possible.

Both of these properties are checked only once, at startup.

Proxies

A proxy server allows indirect connection to network services and
is used mainly for security (to get through firewalls) and
performance reasons (proxies often do provide caching mechanisms).
The following properties allow for configuration of the various type
of proxies.

HTTP

The following proxy settings are used by the HTTP protocol handler.

http.proxyHost (default: <none>)
The hostname, or address, of the proxy server

http.proxyPort (default: 80)
The port number of the proxy server.

http.nonProxyHosts (default: localhost|127.*|[::1])
Indicates the hosts that should be accessed without going
through the proxy. Typically this defines internal hosts.
The value of this property is a list of hosts,
separated by the '|' character. In addition the wildcard
character '*' can be used for pattern matching. For example
-Dhttp.nonProxyHosts=”*.foo.com|localhost”
will indicate that every hosts in the foo.com domain and the
localhost should be accessed directly even if a proxy server is
specified.

The default value excludes all common variations of the loopback address.

HTTPSThis is HTTP over SSL, a secure version of HTTP
mainly used when confidentiality (like on payment sites) is needed.

The following proxy settings are used by the HTTPS protocol handler.

https.proxyHost(default: <none>)
The hostname, or address, of the proxy server

https.proxyPort (default: 443)
The port number of the proxy server.

The HTTPS protocol handler will use the same nonProxyHosts
property as the HTTP protocol.

FTP

The following proxy settings are used by the FTP protocol handler.

ftp.proxyHost(default: <none>)
The hostname, or address, of the proxy server

ftp.proxyPort (default: 80)
The port number of the proxy server.

ftp.nonProxyHosts (default: localhost|127.*|[::1])
Indicates the hosts that should be accessed without going
through the proxy. Typically this defines internal hosts.
The value of this property is a list of hosts, separated by
the '|' character. In addition the wildcard character
'*' can be used for pattern matching. For example
-Dhttp.nonProxyHosts=”*.foo.com|localhost”
will indicate that every hosts in the foo.com domain and the
localhost should be accessed directly even if a proxy server is
specified.

The default value excludes all common variations of the loopback address.

SOCKSThis is another type of proxy. It allows for lower
level type of tunneling since it works at the TCP level. In effect,
in the Java(tm) platform setting a SOCKS proxy server will result in
all TCP connections to go through that proxy, unless other proxies
are specified. If SOCKS is supported by a Java SE implementation, the
following properties will be used:

socksProxyHost (default: <none>)
The hostname, or address, of the proxy server.

socksProxyPort (default: 1080)
The port number of the proxy server.

socksProxyVersion (default: 5)
The version of the SOCKS protocol supported by the server. The
default is 5 indicating SOCKS V5, alternatively
4 can be specified for SOCKS V4. Setting the property
to values other than these leads to unspecified behavior.

java.net.socks.username (default: <none>)
Username to use if the SOCKSv5 server asks for authentication
and no java.net.Authenticator instance was found.

java.net.socks.password (default: <none>)
Password to use if the SOCKSv5 server asks for authentication
and no java.net.Authenticator instance was found.

Note that if no authentication is provided with either the above
properties or an Authenticator, and the proxy requires one, then
the user.name property will be used with no password.

java.net.useSystemProxies (default: false)
On recent Windows systems and on Gnome 2.x systems it is possible to
tell the java.net stack, setting this property to true, to use
the system proxy settings (both these systems let you set proxies
globally through their user interface). Note that this property is
checked only once at startup.

Misc HTTP properties

http.agent (default: “Java/<version>”)
Defines the string sent in the User-Agent request header in http
requests. Note that the string “Java/<version>” will
be appended to the one provided in the property (e.g. if
-Dhttp.agent=”foobar” is used, the User-Agent header will
contain “foobar Java/1.5.0” if the version of the VM is
1.5.0). This property is checked only once at startup.

http.keepalive (default: true)
Indicates if persistent connections should be supported. They improve
performance by allowing the underlying socket connection to be reused
for multiple http requests. If this is set to true then persistent
connections will be requested with HTTP 1.1 servers.

http.maxConnections (default: 5)
If HTTP keepalive is enabled (see above) this value determines the
maximum number of idle connections that will be simultaneously kept
alive, per destination.

http.maxRedirects (default: 20)
This integer value determines the maximum number, for a given request,
of HTTP redirects that will be automatically followed by the
protocol handler.

http.auth.digest.validateServer (default: false)

http.auth.digest.validateProxy (default: false)

http.auth.digest.cnonceRepeat (default: 5)

These 3 properties modify the behavior of the HTTP digest
authentication mechanism. Digest authentication provides a limited
ability for the server to authenticate itself to the client (i.e.
By proving it knows the user's password). However not all HTTP
servers support this capability and by default it is turned off. The
first two properties can be set to true to enforce this check for
authentication with either an origin or proxy server, respectively.

It is usually not necessary to change the third property. It
determines how many times a cnonce value is re-used. This can be
useful when the MD5-sess algorithm is being used. Increasing this
value reduces the computational overhead on both client and server
by reducing the amount of material that has to be hashed for each
HTTP request.

http.auth.ntlm.domain (default: <none>)
NTLM is another authentication scheme. It uses the
java.net.Authenticator class to acquire usernames and passwords when
they are needed. However NTLM also needs the NT domain name. There are
3 options for specifying that domain:

Do not specify it. In some environments the domain is
actually not required and the application does not have to specify
it.

The domain name can be encoded within the username by
prefixing the domain name, followed by a back-slash '\' before the
username. With this method existing applications that use the
authenticator class do not need to be modified, as long as users
are made aware that this notation must be used.

If a domain name is not specified as in method 2) and these
property is defined, then its value will be used a the domain
name.

All these properties are checked only once at startup.

Address Cache

The java.net package, when doing name resolution, uses an address
cache for both security and performance reasons. Any address
resolution attempt, be it forward (name to IP address) or reverse (IP
address to name), will have its result cached, whether it was
successful or not, so that subsequent identical requests will not
have to access the naming service. These properties allow for some
tuning on how the cache is operating.

networkaddress.cache.ttl (default: see below)
Value is an integer corresponding to the number of seconds successful
name lookups will be kept in the cache. A value of -1, or any other
negative value for that matter, indicates a “cache forever”
policy, while a value of 0 (zero) means no caching. The default value
is -1 (forever) if a security manager is installed, and implementation
specific when no security manager is installed.

networkaddress.cache.negative.ttl (default: 10)
Value is an integer corresponding to the number of seconds an
unsuccessful name lookup will be kept in the cache. A value of -1,
or any negative value, means “cache forever”, while a
value of 0 (zero) means no caching.

Since these 2 properties are part of the security policy, they are
not set by either the -D option or the System.setProperty() API,
instead they are set as security properties.